1. Field of the Invention
The invention relates to fluorescent lamps, and is directed more particularly to a fluorescent lamp having therein means for quenching the arc in the lamp at the end of lamp life.
2. Description of the Prior Art
Fluorescent lamps are increasingly being used with electronic ballasts that operate the lamp at high frequencies. Often such ballasts are of the "instant start" type wherein the open circuit voltage is sufficiently high to ignite the lamp directly, without the need for a separate cathode heating current.
The end of lamp life occurs when one of the electrodes is depleted of its emissive coating. At power line frequencies and with low open circuit voltage ballasts, the lamp arc is passively extinguished when the first electrode fails. However, in the case of electronic instant start ballasts, the lamp arc does not necessarily extinguish when the first electrode fails. The open circuit voltage provided by instant start ballasts is sufficiently high to cause the lamp to continue to operate in a "cold cathode" mode. During cold cathode operation, the cathode voltage rises from around 12 volts to 50 volts, or higher.
Referring to FIGS. 1 and 2, in a lamp 2 having electrodes 4, 6 at either end of a glass tube 8, respectively, upon failure of the first electrode 6, ion bombardment heats the tungsten coil 10, lead wires 12, 14, and any other electrically connected metallic structures within the glass tube 8. The heating of the metallic components is to such a high temperature that the components provide sufficient thermionic and secondary electron emissions to sustain the arc. Wattage dissipation in the failed lamp end greatly increases. As a result, the end of the tube 8 heats far above its normal operating temperature. The lead wires 12, 14 within the envelope 8 often become molten and melt through the envelope and/or cause the envelope to crack and sometimes break upon removal of the lamp from a fixture. The excessive heating of the lamp end can also cause damage to a socket or lamp fixture in which the lamp is mounted, or melting of a plastic lamp base 16.
To alleviate the problem, instant start electronic ballasts have been designed with additional circuitry to sense a rise in lamp voltage, or other events occurring upon cathode depletion, and shut down the system. However, such additional electronic components significantly increase the cost of the ballast. Further, many ballasts which do not include such a feature already exist in present lamp installations.
Accordingly, there exists a need for a fluorescent lamp which self-contains means for arc shut-down at the end of life of the lamp, which shut-down means does not include or require additional circuitry or electronic components.